The present invention relates to a lithium ion secondary battery having an anode comprised of substantially crystalline graphitic carbon nanofibers composed of graphite sheets. The graphite sheets are substantially perpendicular or parallel to the longitudinal axis of the carbon nanofiber. This invention also relates to the above-mentioned electrode for use in lithium ion secondary batteries.
Lithium ion secondary batteries are currently the leading portable energy storage device for the consumer electronics market. They are comprised of selected carbon materials as an anode, a lithium transition metal oxide such as LiCoO2, LiNiO2 or LiMnO2 as the cathode, and an electrolyte typically comprised of a lithium salt in an organic solvent. The ability to intercalate lithium into the carbon structure of the anode usually determines the performance of the battery. The performance of lithium ion secondary batteries, such as the charge/discharge capacity, voltage profile and cyclic stability, strongly depends on the microstructure of the carbon anode material. Types of carbon materials that have been investigated for use in lithium ion batteries include both graphitic carbons and non-graphitic carbons, such as semi-coke and glass-like carbons. Graphites and graphitized soft carbons have been studied the most because of their desirable high volumetric reversible capacity and their low electrode potential relative to lithium metal. Graphite materials are preferred because of their ability to intercalate lithium. When graphite is used as an anode-active material, the quantity of lithium inserted between the layers is typically one lithium atom to six carbon atoms. Thus, the theoretical capacity of carbon, per unit weight, is 372 mAh/g.
Lithium ions move back and forth between electrodes during the charging and discharging processes of the battery. During the battery charging process, Li ions from the cathode move through an electrolyte, collect an electron and proceed to intercalate within the carbon structure of the anode. The opposite reaction takes place during discharging, i.e. the neutral Li ions deintercalates, loses an electron to form Li+, and diffuse towards the cathode. Useful work is obtained by circulating the electron through an external circuit. The oxide lattice of the cathode captures the electron where the transition metal oxide undergoes reduction. Equation 1 below illustrates the charge/discharge total reaction of a Li-Ion battery. The half reaction occurring at the anode, that is based on Li+ intercalation and deintercalation of carbon, is shown in Equation 2 below:                               LiCoO          2                +                  y          ⁢                      xe2x80x83                    ⁢                      C            ⁢                          xe2x80x83                        ⁢                          ⟷              Discharge              Charge                        ⁢                          xe2x80x83                        ⁢                          Li                              l                -                x                                              ⁢                      CoO            2                          +                              Li            x                    ⁢                      C            y                                              Equation        ⁢                  xe2x80x83                ⁢        1                                          C          y                +                  x          ⁢                      xe2x80x83                    ⁢          Li                ⁢                  xe2x80x83                +                  x          ⁢                      xe2x80x83                    ⁢                                    e              -                        ⁢                          xe2x80x83                        ⁢                          ⟷              Discharge              Charge                        ⁢                          xe2x80x83                        ⁢                          Li              x                                ⁢                      C            y                                              Equation        ⁢                  xe2x80x83                ⁢        2            
Carbon materials suggested for use as an anode for batteries are disclosed in Recent Trends in Carbon Negative Electrode Materials, T. Kasuh et al., Journal of Power Sources 68 (1997) pages 99-101; The Basic Electroanalytical Behavior of Practical Graphite-Lithium Intercalation Electrodes, B. Markovsky et al., Electrochimica Acta, Vol. 43, Nos. 16 and 17, (1998) pages 2287-2304; On the Choice of Graphite for Lithium Ion Batteries, B. Simon, Journal of Power Sources 81-82, (1999) pages 312-316; A Study of Highly Oriented Pyrolytic Graphite as a Model for the Graphite Anode in Li-Ion Batteries, Journal of the Electrochemical Society, 146 (3), (1999) pages 824-832; Characteristics of Coke Carbon Modified with Mesophase-Pitch as a Negative Electrode for Lithium Ion Batteries, Y. Sato et al., Journal of Power Sources 81-82, (1999) pages 182-186; Coke vs. Graphite as Anodes for Lithium-Ion Batteries, Hang Shi, Journal of Power Sources 75, (1998) pages 64-72. All of these articles are incorporated herein by reference. The carbon materials disclosed in the above publications do not exhibit a sufficient capacity, in the potential range, for use as anodes in commercial batteries. One reason for this is an undesirable linear increase in potential during the deintercalation of lithium. This is true even if the carbon material has a certain capacity, as seen from cokes, when used as an electrode material.
Also, when an electrode is manufactured using a carbon material, bulk density is an important factor. Japanese Laid-Open Patent No. 63-230512 teaches that a powdered graphite cannot provide sufficient capacity as an active material in Li-Ion batteries because the degree of crystalline perfection is not sufficiently high.
Recently, nanocarbons, such as multi-walled carbon nanotubes and fibrils have been suggested for use in lithium ion batteries. U.S. Pat. No. 5,879,836 teaches the use of fibrils as the material for the lithium ion battery anode. Fibrils are described, in that patent, as being composed of parallel layers of carbon in the form of a series of concentric tubes disposed about the longitudinal axis, rather than as multi-layers of flat graphite sheets, as in the carbon nanofibers used in the present invention. Carbon fibrils are similar in structure to the so-called xe2x80x9cbuckytubesxe2x80x9d, or nanotubes, that are described in an article entitled Fullerenes, M. S. Dresselhaus, et al, Journal of Materials Research, Vol. 8, No. 8, August 1993, pages 2087-2092, and is incorporated herein by reference. Fullerenes are Cn cage molecules built from a collection of hexagonal and pentagonal faces. The C60-derived tubules can be defined, in simplest terms, by bisecting a C60 molecule at the equator and joining the two resulting hemispheres with a cylindrical tube, one monolayer thick and having the same diameter as the C60 molecule. Carbon nanotubes can also be defined as substantially hollow structures comprised of substantially parallel graphite circular layers aligned at distances of about 0.335 nm to 0.67 nm from each other.
While lithium ion batteries have met with some commercial success using conventional carbon materials, there remains a need for lithium-ion batteries that can achieve a higher level of performance than those currently available.
In accordance with the present invention, there is provided a lithium ion battery comprising an anode, a cathode and an electrolyte, wherein the anode is comprised of substantially crystalline graphitic carbon nanofibers comprised of graphite sheets that are aligned in directions that are perpendicular or parallel to the longitudinal axis of the nanofibers, wherein the distance between graphite sheets is from about 0.335 nm to about 0.67 nm, and having a crystallinity greater than about 95%.
In a preferred embodiment of the present invention, the graphite sheets are substantially perpendicular to the longitudinal axis of the nanofiber.
In another preferred embodiment of the present invention, the nanofiber is one wherein the distance between the graphite sheets is from about 0.335 and 0.40 nm
In yet another preferred embodiment of the present invention, the cathode is comprised of a lithium transition oxide selected from the group consisting of LiCoO2, LiNiO2 and LiMnO2.
In still another preferred embodiment of the present invention, the degree of crystallinity of the nanofibers are at least about 98%.